The invention relates to a process for electrolytic demetallization of a solution containing metal ions where an electrical d.c. current is controlled between an anode submerged in the solution and a cathode by means of a programmable control unit in such a way that the current intensity is reduced in steps to an approximately constant value during sequentially timed intervals until the remaining concentration of the metal ions falls below a predetermined value, as well as to an apparatus for current control in an electrolytic metal depletion cell.
A process for electrolytic desilverization of photographic fixing baths is known from DE-OS 28 08 095, and U.S. Pat. No. 4,139,431 in which the solution to be desilvered is circulated in a closed cycle from a reservoir through an electrolysis cell and back again, where the volume of the cell and the circulated amount per unit of time are selected or adjusted in such a way that the dwell time of the electrolyte in the electrolysis cell is at most one minute. The electrode load is reduced in sequentially timed steps; in the course of this, a pH-adjusted redox system is supplied to the solution to be desilvered. In an apparatus for executing the process, a constant current is reduced in steps in sequentially timed intervals until the remaining concentration falls below a predetermined value. In this manner the electrode load can be applied by means of a preprogrammed automatic control with the aid of a programmable control in which all or at least a part of the values to be controlled are automatically controlled in accordance with a preselectable program.
In this connection it has proven to be problematical that this is a mere control apparatus without any comparison measurements between a desired set value and a measured actual value. Optimization of the electro-chemical process with maximum energy yield is not possible because of the mechanical control.
An electrode potential control for electrolysis in connection with the retrieval of silver from photographic solutions is furthermore known from DE-OS 39 22 959, Dittrich et al., where an electrode potential of the working cathode affected by the electrolysis current is measured by means of a reference electrode.
This is a continuous potentio-static control, the electrolyte throughput amount of which is comparatively small because of its system-defined limit current; additionally, such a control can only be used in a device of one anode and one cathode each, because the pair of measuring electrodes only provides the values for a single cathode.
It is the object of an invention to provide a process for automatic current control of metal depletion cells while using as high as possible a current density; it is furthermore intended to create an autonomous apparatus with a control unit taking into consideration the progress of the electro-chemical process, which reduces the cell current in steps; in addition it is intended to use the automatic current control also with cells having a plurality of cathodes placed opposite of one or two anodes.
The set value of the current is preferably reduced by an amount which in relation to the preset value lies in the range of between 1:1.5 to 1:7. In this case the solution is supplied to the metal depletion cell in batches.
In a preferred embodiment of the apparatus, the electronic evaluation device has a monostable trigger circuit for the pulse generation; the input of the set value set point adjuster is provided with a pulse counter, where each count of the counter corresponds to a predetermined set value of the current.
The large energy yield and the short metal extraction period prove to be advantageous.